WBI/Britt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WBI/Britt clear sky chart from 2005-11-06 to 2024-01-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	17% 10% 4 7% 5% 3 5% 4 5% 7% 34%	74721
	Night	17% 12% 4 7% 6% 3 4 4 5% 6% 31%	75097
Transparency	Day	50% 6% 13% 20% 10% 1	74424
	Night	54% 6% 11% 18% 9% 1	74930
Seeing	Day	21% 14% 31% 23% 9% 2	14010
	Night	22% 12% 29% 25% 10% 1	23306

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	17% 18% 8% 11% 6% 4 4 4 5% 6% 18%	1328
	11th to 20th	Day	16% 17% 7% 10% 7% 3 5% 3 4 7% 22%	1416
	21st to end	Day	17% 14% 6% 11% 6% 4 5% 4 4 5% 24%	1782
February	1st to 10th	Day	13% 19% 6% 10% 6% 3 6% 5% 5% 6% 22%	1620
	11th to 20th	Day	14% 14% 7% 8% 7% 4 6% 3 4 5% 28%	1573
	21st to end	Day	15% 14% 6% 11% 7% 3 5% 4 4 5% 25%	1390
March	1st to 10th	Day	14% 11% 5% 7% 6% 4 5% 5% 5% 5% 33%	1928
	11th to 20th	Day	14% 11% 4 8% 6% 3 5% 4 5% 5% 34%	1980
	21st to end	Day	17% 10% 4 7% 5% 2 4 3 4 6% 38%	2178
April	1st to 10th	Day	21% 11% 4 7% 5% 2 5% 5% 5% 7% 29%	2099
	11th to 20th	Day	21% 8% 3 5% 5% 2 3 4 4 7% 38%	2303
	21st to end	Day	21% 10% 4 5% 5% 2 4 4 4 5% 37%	2340
May	1st to 10th	Day	18% 8% 3 6% 5% 3 4 4 5% 7% 37%	2343
	11th to 20th	Day	17% 8% 3 5% 5% 3 4 4 5% 7% 38%	2478
	21st to end	Day	14% 5% 2 5% 5% 3 4 4 6% 8% 44%	2725
June	1st to 10th	Day	16% 6% 3 6% 6% 3 5% 5% 6% 7% 38%	2686
	11th to 20th	Day	13% 6% 2 5% 5% 4 5% 5% 6% 8% 42%	2700
	21st to end	Day	16% 7% 2 6% 5% 3 5% 5% 5% 8% 37%	2700
July	1st to 10th	Day	10% 4 2 5% 5% 3 5% 4 6% 8% 48%	2700
	11th to 20th	Day	11% 5% 2 6% 5% 3 5% 5% 6% 8% 44%	2700
	21st to end	Day	11% 5% 3 5% 5% 3 5% 5% 6% 9% 44%	2628
August	1st to 10th	Day	12% 6% 2 5% 5% 3 4 4 6% 8% 44%	2327
	11th to 20th	Day	12% 5% 3 6% 5% 4 5% 5% 5% 8% 41%	2303
	21st to end	Day	13% 6% 3 6% 6% 3 4 5% 5% 7% 41%	2550
September	1st to 10th	Day	14% 6% 3 5% 6% 3 4 5% 6% 8% 40%	2160
	11th to 20th	Day	16% 5% 2 5% 5% 3 4 5% 5% 8% 43%	2006
	21st to end	Day	20% 6% 3 6% 5% 3 5% 4 5% 6% 36%	1976
October	1st to 10th	Day	19% 7% 3 7% 6% 3 5% 4 5% 7% 34%	1800
	11th to 20th	Day	27% 9% 4 7% 6% 4 4 5% 4 5% 25%	1800
	21st to end	Day	27% 12% 4 7% 5% 3 4 4 4 6% 23%	1913
November	1st to 10th	Day	19% 14% 3 6% 6% 3 5% 5% 4 7% 28%	1555
	11th to 20th	Day	19% 15% 4 9% 7% 3 5% 4 5% 5% 24%	1520
	21st to end	Day	24% 19% 4 9% 6% 3 4 4 3 4 19%	1520
December	1st to 10th	Day	17% 22% 8% 8% 5% 2 4 4 4 6% 19%	1520
	11th to 20th	Day	20% 20% 5% 9% 5% 3 4 3 4 4 24%	1743
	21st to end	Day	24% 21% 6% 11% 6% 4 3 3 3 4 14%	2431
January	1st to 10th	Night	17% 18% 8% 10% 7% 4 4 4 4 5% 20%	2838
	11th to 20th	Night	19% 17% 7% 10% 7% 4 4 4 4 4 21%	2684
	21st to end	Night	14% 14% 7% 10% 7% 4 6% 3 4 5% 26%	2574
February	1st to 10th	Night	13% 16% 6% 11% 7% 3 5% 4 5% 5% 24%	2340
	11th to 20th	Night	13% 14% 6% 10% 6% 4 4 4 4 5% 28%	2273
	21st to end	Night	15% 14% 5% 9% 8% 4 5% 5% 4 5% 24%	1862
March	1st to 10th	Night	13% 13% 5% 7% 6% 3 5% 4 4 7% 35%	2018
	11th to 20th	Night	16% 9% 4 7% 6% 3 5% 4 5% 6% 34%	1979
	21st to end	Night	15% 9% 4 6% 5% 3 4 4 5% 6% 38%	2178
April	1st to 10th	Night	22% 11% 4 7% 5% 3 5% 4 4 6% 30%	1860
	11th to 20th	Night	19% 8% 4 5% 5% 3 4 4 4 6% 38%	1620
	21st to end	Night	20% 10% 4 7% 7% 3 4 3 5% 6% 31%	1620
May	1st to 10th	Night	15% 7% 3 6% 5% 3 6% 5% 6% 8% 37%	1616
	11th to 20th	Night	14% 6% 3 4 5% 2 4 6% 7% 8% 39%	1415
	21st to end	Night	10% 5% 3 6% 6% 3 5% 6% 7% 7% 43%	1550
June	1st to 10th	Night	14% 7% 3 6% 5% 3 6% 5% 5% 7% 39%	1271
	11th to 20th	Night	13% 4 2 4 5% 2 4 5% 6% 9% 45%	1260
	21st to end	Night	13% 6% 3 6% 6% 3 4 4 6% 7% 41%	1260
July	1st to 10th	Night	10% 4 3 4 4 2 5% 5% 6% 9% 49%	1260
	11th to 20th	Night	10% 3 3 5% 5% 3 5% 5% 7% 7% 47%	1260
	21st to end	Night	10% 5% 2 7% 4 2 5% 5% 5% 9% 46%	1684
August	1st to 10th	Night	11% 6% 3 5% 5% 3 4 5% 5% 8% 46%	1611
	11th to 20th	Night	12% 5% 2 5% 5% 3 4 5% 5% 7% 47%	1598
	21st to end	Night	14% 7% 3 4 5% 4 4 3 6% 6% 44%	1786
September	1st to 10th	Night	12% 4 2 6% 4 4 5% 5% 5% 7% 46%	1800
	11th to 20th	Night	13% 5% 2 5% 5% 2 4 4 4 8% 47%	1952
	21st to end	Night	17% 7% 2 6% 4 3 4 4 5% 8% 40%	1983
October	1st to 10th	Night	21% 6% 3 6% 6% 3 5% 4 4 6% 36%	2160
	11th to 20th	Night	27% 10% 3 6% 5% 3 4 4 4 7% 27%	2160
	21st to end	Night	28% 12% 4 7% 5% 3 3 3 4 6% 25%	2438
November	1st to 10th	Night	22% 12% 4 6% 5% 2 3 4 4 7% 32%	2521
	11th to 20th	Night	19% 13% 5% 8% 7% 3 4 5% 5% 5% 26%	2660
	21st to end	Night	24% 17% 5% 8% 5% 3 5% 5% 5% 5% 18%	2660
December	1st to 10th	Night	19% 21% 8% 8% 5% 3 4 4 4 4 18%	2660
	11th to 20th	Night	21% 18% 5% 9% 6% 3 4 4 4 5% 22%	3450
	21st to end	Night	23% 21% 6% 12% 6% 3 3 3 3 4 16%	5236

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	68% 7% 10% 12% 3	1329
	11th to 20th	Day	63% 6% 9% 13% 7% 1	1416
	21st to end	Day	64% 5% 7% 14% 9% 1	1782
February	1st to 10th	Day	63% 5% 11% 14% 6% 1	1620
	11th to 20th	Day	60% 4 10% 16% 10% 1	1582
	21st to end	Day	61% 4 12% 16% 7%	1390
March	1st to 10th	Day	52% 5% 16% 14% 11% 1	1931
	11th to 20th	Day	51% 4 12% 21% 10% 2	1980
	21st to end	Day	48% 4 10% 21% 13% 4	2178
April	1st to 10th	Day	53% 6% 13% 16% 10% 1	2099
	11th to 20th	Day	47% 4 9% 24% 14% 1	2303
	21st to end	Day	50% 5% 10% 19% 13% 3	2340
May	1st to 10th	Day	46% 5% 11% 21% 14% 3	2343
	11th to 20th	Day	44% 5% 11% 24% 14% 2	2482
	21st to end	Day	38% 8% 15% 23% 13% 2	2725
June	1st to 10th	Day	44% 6% 12% 23% 13% 1	2686
	11th to 20th	Day	38% 7% 12% 25% 16% 1	2700
	21st to end	Day	44% 7% 14% 22% 11% 1	2700
July	1st to 10th	Day	32% 7% 17% 27% 15% 2	2626
	11th to 20th	Day	36% 7% 15% 31% 10% 1	2619
	21st to end	Day	37% 7% 15% 28% 12% 1	2482
August	1st to 10th	Day	36% 6% 19% 27% 11% 1	2314
	11th to 20th	Day	38% 9% 18% 25% 10%	2303
	21st to end	Day	41% 7% 17% 25% 9%	2550
September	1st to 10th	Day	41% 6% 15% 26% 11% 1	2160
	11th to 20th	Day	38% 7% 16% 30% 9%	2006
	21st to end	Day	48% 6% 15% 24% 6%	1976
October	1st to 10th	Day	48% 8% 14% 20% 10%	1800
	11th to 20th	Day	61% 7% 13% 12% 6%	1800
	21st to end	Day	63% 6% 13% 12% 6%	1913
November	1st to 10th	Day	56% 9% 12% 19% 5%	1555
	11th to 20th	Day	62% 8% 11% 13% 6%	1520
	21st to end	Day	71% 7% 10% 9% 4	1520
December	1st to 10th	Day	67% 9% 9% 10% 3 1	1520
	11th to 20th	Day	66% 5% 9% 15% 4	1743
	21st to end	Day	75% 5% 8% 10% 2	2431
January	1st to 10th	Night	68% 6% 9% 11% 6%	2838
	11th to 20th	Night	66% 5% 10% 11% 6% 1	2684
	21st to end	Night	61% 5% 7% 15% 10% 1	2574
February	1st to 10th	Night	61% 6% 11% 15% 6% 1	2340
	11th to 20th	Night	58% 5% 12% 17% 7% 1	2275
	21st to end	Night	60% 5% 13% 14% 7% 1	1862
March	1st to 10th	Night	49% 5% 13% 17% 14% 2	2028
	11th to 20th	Night	50% 5% 12% 19% 11% 2	1980
	21st to end	Night	46% 5% 11% 19% 15% 4	2178
April	1st to 10th	Night	55% 4 9% 18% 12% 2	1860
	11th to 20th	Night	48% 4 7% 22% 16% 2	1620
	21st to end	Night	54% 5% 9% 15% 15% 2	1620
May	1st to 10th	Night	44% 7% 10% 21% 14% 3	1616
	11th to 20th	Night	40% 7% 8% 25% 16% 3	1416
	21st to end	Night	36% 9% 15% 21% 17% 3	1551
June	1st to 10th	Night	44% 6% 11% 22% 14% 3	1271
	11th to 20th	Night	35% 7% 13% 24% 20% 1	1260
	21st to end	Night	40% 7% 16% 22% 14% 2	1260
July	1st to 10th	Night	29% 5% 15% 29% 18% 3	1225
	11th to 20th	Night	35% 7% 13% 28% 15% 2	1218
	21st to end	Night	36% 7% 15% 28% 14% 1	1591
August	1st to 10th	Night	36% 7% 18% 27% 12% 1	1600
	11th to 20th	Night	35% 7% 18% 27% 12% 1	1598
	21st to end	Night	40% 6% 15% 26% 12% 1	1786
September	1st to 10th	Night	36% 8% 14% 27% 14% 2	1800
	11th to 20th	Night	37% 6% 15% 31% 10%	1952
	21st to end	Night	42% 7% 15% 27% 8%	1983
October	1st to 10th	Night	49% 8% 10% 19% 12%	2160
	11th to 20th	Night	59% 7% 11% 18% 5%	2160
	21st to end	Night	63% 6% 10% 14% 7% 1	2438
November	1st to 10th	Night	54% 7% 13% 20% 6%	2521
	11th to 20th	Night	61% 8% 11% 14% 5% 1	2660
	21st to end	Night	67% 10% 10% 10% 4	2660
December	1st to 10th	Night	71% 8% 7% 10% 3	2660
	11th to 20th	Night	66% 7% 9% 12% 6%	3449
	21st to end	Night	74% 4 11% 8% 3	5236

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 23% 29% 18% 6% 1	180
	11th to 20th	Day	24% 12% 34% 23% 6% 1	217
	21st to end	Day	28% 13% 36% 16% 7% 1	264
February	1st to 10th	Day	31% 26% 29% 10% 3	240
	11th to 20th	Day	22% 24% 40% 12% 2	240
	21st to end	Day	32% 27% 29% 11% 1	198
March	1st to 10th	Day	22% 20% 33% 20% 4	371
	11th to 20th	Day	22% 17% 36% 19% 5% 1	420
	21st to end	Day	26% 18% 32% 18% 5% 1	462
April	1st to 10th	Day	25% 15% 33% 19% 8%	420
	11th to 20th	Day	30% 17% 24% 17% 7% 4	412
	21st to end	Day	27% 17% 30% 18% 8%	420
May	1st to 10th	Day	24% 10% 35% 22% 7% 2	423
	11th to 20th	Day	19% 14% 33% 21% 10% 3	447
	21st to end	Day	18% 12% 32% 24% 12% 2	485
June	1st to 10th	Day	17% 14% 35% 26% 7% 1	616
	11th to 20th	Day	14% 12% 35% 26% 10% 2	630
	21st to end	Day	19% 9% 35% 27% 8% 2	630
July	1st to 10th	Day	10% 7% 35% 34% 13% 2	621
	11th to 20th	Day	12% 9% 32% 32% 13% 3	618
	21st to end	Day	11% 7% 32% 33% 14% 4	506
August	1st to 10th	Day	17% 5% 27% 31% 13% 6%	446
	11th to 20th	Day	13% 4 26% 31% 16% 9%	439
	21st to end	Day	16% 13% 25% 28% 12% 5%	495
September	1st to 10th	Day	15% 12% 26% 29% 13% 4	450
	11th to 20th	Day	17% 8% 25% 31% 14% 4	450
	21st to end	Day	24% 10% 24% 24% 15% 3	450
October	1st to 10th	Day	24% 12% 28% 23% 11% 2	450
	11th to 20th	Day	29% 17% 31% 18% 4 1	450
	21st to end	Day	34% 18% 29% 16% 2 1	428
November	1st to 10th	Day	28% 20% 35% 14% 3	214
	11th to 20th	Day	22% 27% 35% 13% 3 1	180
	21st to end	Day	37% 21% 23% 15% 3 1	180
December	1st to 10th	Day	28% 16% 36% 17% 4	180
	11th to 20th	Day	32% 22% 32% 10% 3 1	180
	21st to end	Day	28% 25% 30% 13% 3	198
January	1st to 10th	Night	26% 15% 31% 22% 5% 1	950
	11th to 20th	Night	29% 15% 30% 20% 5%	919
	21st to end	Night	20% 11% 33% 26% 9% 1	792
February	1st to 10th	Night	21% 20% 34% 21% 4	720
	11th to 20th	Night	20% 15% 36% 23% 6%	703
	21st to end	Night	23% 17% 34% 24% 3	592
March	1st to 10th	Night	20% 16% 37% 23% 5%	720
	11th to 20th	Night	23% 16% 33% 21% 7% 1	720
	21st to end	Night	20% 12% 34% 27% 7% 1	792
April	1st to 10th	Night	25% 14% 32% 22% 6%	720
	11th to 20th	Night	24% 12% 27% 25% 10% 1	556
	21st to end	Night	24% 11% 27% 26% 11%	540
May	1st to 10th	Night	18% 6% 28% 36% 12% 1	536
	11th to 20th	Night	17% 7% 29% 32% 15% 1	354
	21st to end	Night	11% 8% 32% 35% 14% 1	389
June	1st to 10th	Night	16% 8% 28% 34% 13% 1	360
	11th to 20th	Night	17% 8% 24% 36% 16% 1	360
	21st to end	Night	16% 6% 22% 35% 19% 2	360
July	1st to 10th	Night	11% 5% 23% 42% 18% 1	350
	11th to 20th	Night	11% 5% 26% 34% 23% 1	348
	21st to end	Night	11% 6% 24% 36% 21% 1	525
August	1st to 10th	Night	11% 4 24% 37% 21% 4	537
	11th to 20th	Night	13% 5% 22% 32% 25% 4	534
	21st to end	Night	17% 6% 24% 29% 22% 2	598
September	1st to 10th	Night	13% 8% 27% 29% 20% 3	720
	11th to 20th	Night	16% 10% 23% 29% 19% 4	720
	21st to end	Night	20% 9% 23% 28% 16% 3	720
October	1st to 10th	Night	23% 10% 29% 24% 12% 2	720
	11th to 20th	Night	31% 13% 28% 21% 6% 1	720
	21st to end	Night	33% 17% 28% 19% 3 1	792
November	1st to 10th	Night	29% 13% 29% 22% 7%	740
	11th to 20th	Night	25% 19% 30% 20% 6%	760
	21st to end	Night	33% 16% 29% 17% 4	760
December	1st to 10th	Night	32% 16% 33% 16% 4	760
	11th to 20th	Night	30% 19% 32% 15% 4 1	874
	21st to end	Night	32% 16% 30% 18% 3	1045

Page generated at 2024-01-14 09:54:59. Version ac07

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